In the hydrocarbon exploration industry, remote sensing of underground geological formations using seismic waves provides information on the location, shape, and rock and fluid properties of potential hydrocarbon reservoirs. The standard technique comprises the activation of a source of acoustic energy which radiates seismic waves into the earth. These seismic waves reflect from and refract through subsurface geologic layers (acoustic illumination or insonification). The recording of these seismic waves by many different receivers (pressure or motion sensors) are ideally situated so as to optimize the ratio of information obtained to cost. This basic sourcing/insonification/recording procedure is repeated many times at slightly different locations over a subsurface region of interest.
However, the resolution required of the seismic data for a detailed interpretation and adequate risk reduction can be suboptimal given the cost constraints inherent in seismic acquisition. Methods have been taught using generally simultaneously fired energy sources in an effort to obtain more information for a given cost.
Edington, U.S. Pat. No. 4,953,657 teaches a method of time delay source coding. In this method “a series of shots is made at each shotpoint with a determinable time delay between the activation of each source for each shot”.
The “series of shots” refers to occupying each shotpoint location for several consecutive shots. This methodology may be acceptable for seismic acquisition on land where seismic sources can easily remain fixed at one shot location for an indefinite time. However, the method is not well suited for marine recording in which a seismic receiver cable is being towed behind a boat. A certain minimum velocity is necessary to preserve the approximately linear trajectory of the cable.
De Kok et. al, U.S. Pat. No. 6,545,944, teaches a method for acquiring and processing seismic data from several simultaneously activated sources. In particular, the method requires that several independently controllable “source elements” be activated in a fixed sequence, at successive neighboring locations. This activation sequence unavoidably smears the energy from a single effective source across several neighboring shot locations, necessitating an interpolation step and the introduction of unwanted interpolation noise. Further, the success of building an effective source by spatial sequencing of source sub-elements appears to depend sensitively on source timing precision and sea-state.
Beasley et al., U.S. Pat. No. 5,924,049 also teaches a method of acquiring and processing seismic data using several separate sources. In the preferred embodiment, it teaches that the sources can be activated sequentially with a constant inter-source time delay (up to 15 and 20 seconds). During the processing stage, the method requires anywhere from 2% to 33% of data overlap between panels of data from different sources. Further, it relies on conflicting dips to discriminate energy coming from different source directions, which requires a specific spatial relationship among the sources and the recording cable, and thus is not well suited to simultaneous signals arriving from approximately the same quadrant. In a subsidiary embodiment, the several sources can be activated exactly concurrently, in which case the sources are then arranged to emit signature-encoded wavefields. The decoding and signal separation associated with this type of concurrent signature encoding is usually unsatisfactory. Furthermore, the sources need to be activated at both the leading and trailing ends of the spaced-apart receivers, which is inflexible.
The present invention contrasts with the aforementioned inventions and addresses their shortcomings by teaching a novel way of acquiring and processing seismic data obtained from two or more quasi-simultaneously activated sources.